Rabies virus (Rhabdoviridae: lyssavirus) is a highly pathogenic virus capable of infecting all mammalian species. Rabies is a unique virus - almost invariably fatal, yet preventable. Rabies treatment is highly effective if treatment occurs early. Once signs of clinical illness appear, treatment is considerably less effective, and death usually occurs within two weeks. Rabies is one of the oldest known viral diseases in history, emerging approximately 11,000 years ago possibly as a viral disease in bats (Badrain and Tordo, 2001). If this hypothesis is correct, it is likely that virus-host adaptation has occurred between rabies virus and bats over the past 11,000 years. In the United States, large scale rabies outbreaks have been reported in other vector species such as dogs, skunks, raccoons, and foxes. However, to date, rabies outbreaks have not occurred in bat populations. Contrary to widespread misconception, rabies appears to be maintained in low levels within bat populations ranging from 0.2% to 5.5% (Trimarchi and Debbie, 1977). It is likely that multiple factors influence the ability of rabies virus to be maintained in bat populations, including the circulating rabies virus variant, the homologous (host) species, and the host's ecological niche. Unlike many other viral zoonoses, bat rabies virus variants are typically associated with a distinct species of bat, thus we will refer to these variants as homologous rabies virus variants (Hughes et. al., 2005, Smith, 2002). Interspecies spill-over of rabies virus variants have been reported and are referred to as heterologous virus variants infections. Rabies virus over wintering in bats has been suggested as a method of rabies maintenance in hibernating bat populations (Calisher, 2008), and previous studies have demonstrated that other viruses, such as St. Louis encephalitis virus, may overwinter in bats (Calisher, 2008, Sulkin and Allen, 1974). However, the role of hibernation and maintenance of viral infections in bats have not been well studied. The proposed research will address two major questions, 1) Is the maintenance of rabies virus determined by bat species or the rabies virus variant to which the bat is exposed, and 2) What impact does hibernation have on the maintenance of rabies virus? We will focus on three major studies to provide the basis for determining how rabies virus is maintained in wild bat populations: 1) We will compare the clinical illness in bats exposed to a homologous and heterologous rabies virus variant;2) We will examine the immune response following inoculation with a homologous or heterologous rabies virus variant;and 3) We will elucidate the role that hibernation plays in maintenance of rabies virus. Three species of bats will be included in this proposal: big brown bats (Eptesicus fuscus), little brown bats (Myotis spp), and silver hair bats (Lasionycteris noctivagans). Novel aspects of this study include the investigation of homologous and heterologous rabies virus variants in bats as well as the effect of hibernation on rabies virus maintenance in bats. The overall aim of this proposal is to further our understanding of the interaction between rabies virus and its chiropteran host. In this proposal we outline studies to elucidate the role of the immune system and ecological factors that are unique to bats and determine how they relate to the maintenance of rabies in bat populations. The public health significance of rabies is indisputable since clinical infection generally results in death. This set of studies will provide us with a better understanding of how rabies virus is maintained in the most important rabies vector in the U.S. as well as elucidate the importance of spill over events between bat species. This project will lead to a better understanding of rabies virus maintenance in bat populations and its impact on novel hosts. This project is important to public health as bat rabies variants are capable of infecting, adapting, and spreading in heterologous bat species and terrestrial mammal populations. States with terrestrial rabies have a higher incidence of rabies exposure and increased use of anti-rabies therapeutics. Post exposure prophylaxis should be given as soon as possible following contact with a potentially rabid animal. It is expensive, and it is known to cause significant side effects. Completion of the studies described in this proposal will provide data integral to infectious disease modeling and improve our knowledge of the public health risks associated with bat rabies.